A gateway would need to be created by the dev team of a project. It could net them some of the fees and it could provide a verifiable means of transferring between two assets.
Other than that, Some person, company, or even you could make a verifiable, secure gateway to provide the very features you request. You're gateaway would net fees I think and, if done right, supplement the DEX with additional assets/tokens or whatever you want to call them.
Thinking of the DEX as a framework will help you understand the means of which a coin would be listed on the BitShares exchange.
I do indeed wish you to see your asset of choice -- I think sometime some company will have lots of different , secure and valid projects to the DEX. Only time will tell as it is up to the users and developers in the system.